1. Field of the Invention
The present invention generally relates to a system for generating electrical power. More specifically, in one illustrative example, the invention relates to a local electrical power source for an autonomous subsea installation such as a Christmas tree.
2. Description of the Related Art
The production from a subsea well is controlled by a number of valves that are assembled into a unitary structure generally referred to as a Christmas tree. The actuation of the valves is normally dependent upon hydraulic fluid to power hydraulic actuators that operate the valves. Hydraulic fluid is normally supplied through an umbilical running from a remote station located on a vessel or platform at the surface. Less commonly, the hydraulic umbilical may be run from a land-based station. Usually the actuators are controlled by pilot valves housed in a control module located at or near the subsea installation. The pilot valves direct the supply of fluid to each actuator, as required for each particular operation. The pilot valves may be electrically actuated, such as by solenoids. Such a system is commonly referred to as an electro-hydraulic system.
In addition to the above described flow control valves, actuators, and pilot valves, a number of sensors and detectors are commonly employed in subsea systems to monitor the state of the system and the flow of hydrocarbons from the well. Often a number of sensors, detectors and/or actuators are also located down hole. All these devices are controlled and/or monitored by a dedicated control system, which is usually housed in the control module.
The design of actuators and valves for subsea wells are dictated by stringent safety and reliability standards, because of the danger of uncontrolled release of hydrocarbons. A common requirement is that the valves must be “failsafe close”. In other words, the valves must automatically close upon a loss of power or control, including a failure or malfunction of either the electrical or hydraulic systems. A typical method for providing a failsafe close capability is the use of one or more mechanical springs, which bias the actuator towards the closed position. The hydraulic pressure used to open the valve also holds the springs in the compressed state. Upon a loss of hydraulic pressure, either intentional or due to a system failure, the energy stored in the springs will be released, thus closing the valve. The force required to close a hydraulically actuated valve is dependent upon both the pressure of the fluid controlled by the valve (i.e., the formation pressure), and the ambient pressure (the hydrostatic water pressure for subsea installations) to which the hydraulic actuator is exposed. Higher formation and/or ambient pressures result in larger closing forces, and thus require larger springs.
In many countries there is a requirement for a downhole safety valve (Surface Controlled Subsurface Safety Valve, SCSSV) as an additional safety device for closing the flow path in the well tubing. Because this valve is located in the production flow, it must be operated by hydraulic fluid that is at a higher pressure than the fluid used to actuate the Christmas tree valves. Thus, there is a requirement for an additional system for supplying high-pressure hydraulic fluid to the subsea installation.
In order to control a subsea well, a connection must be established between the well and a monitoring and control station. The monitoring and control station may be located in a platform or floating vessel near the subsea installation, or alternatively in a more remote land station. The connection between the control station and the subsea installation is usually established by installing an umbilical between the two points. The umbilical may include hydraulic lines for supplying hydraulic fluid to the various hydraulic actuators located on or near the well. The umbilical may also include electrical lines for supplying electric power and also for communicating control signals to and/or from the various monitoring and control devices located on or near the well. The typical umbilical is a very complicated and expensive item. The umbilical can cost several thousand U.S. dollars per meter of length, and may be thousands of meters long.
For many years, electric valve actuators have been preferred in land based industries, because electric actuators are more compact than hydraulic actuators. Furthermore, most of the components of a typical electric actuator, such as the electric motor and/or gearbox, are readily available items that can be easily and inexpensively procured from many manufacturers. In some applications, electric actuators are seen as a good alternative to hydraulic actuators because the ambient pressure does not affect the required operating force of an electrically operated valve. Many proposals have been made to use electrically operated actuators instead of hydraulic actuators for subsea deployed valves. Examples of such devices are disclosed in U.S. Pat. Nos. 5,497,672 and 5,984,260. However, because each of these devices incorporates mechanical springs as a failsafe device, these actuators tend to be just as large and bulky as the hydraulic actuators they are intended to replace.
Typically, existing subsea electric actuators are powered from a remote location through a subsea cable, in order to ensure a sufficient and reliable supply of electric power. It is usually required that the power supply be sufficient to operate all the valves simultaneously. In U.S. Pat. Nos. 5,257,549 and 6,595,487 it has been proposed to provide a subsea battery power supply, but only to provide enough emergency power to close a single valve. It has also been proposed to operate a valve in a subsea environment using power generated locally by a thermoelectric device. However, such devices can provide only a limited amount of power, which would not be sufficient to operate all the valves in a larger installation. However, batteries have recently been developed which can store enough power to operate all valves in a subsea installation simultaneously, thus paving the way for solutions where power for the electric motors is stored in locally installed batteries.
Since such a system would have ample locally stored power to close all the valves, the bulky failsafe springs could be eliminated from the actuators. An added advantage is that the operation of such actuators will be independent of the water depth of the system. The need for pilot valves will also be eliminated, since the actuators may be directly controlled electrically. Thus, there will also be potentially large savings on umbilical cost since the hydraulic lines can be removed.
All-electric subsea systems require a more sophisticated control system than electro-hydraulic systems. The control system must control the charging of the batteries and monitor their status. The control system should also monitor the status and position of each valve so that at any time an operator can access this information and intervene if necessary. Furthermore, the control system must implement the failsafe function and close all valves if required.
Under certain circumstances and in certain locations a downhole safety valve (SCSSV) may be required. As discussed above, the low-pressure hydraulic line can be eliminated from the umbilical by using electric actuators for the flow control valves in the tree. In the case where an SCSSV is required, it would obviously be desirable to eliminate the high-pressure line from the umbilical as well. While downhole electric actuators for SCSSV's have been proposed, the hostile downhole environment would render such electric systems unreliable. One possible solution to this dilemma is to provide a local source of high-pressure hydraulic fluid at the subsea well. In this way, a typical hydraulic SCSSV actuator may still be provided downhole, without requiring a hydraulic umbilical to the surface. The local source of high-pressure fluid may be provided by an electrically powered pump or a pressure intensifier, which pressurizes a local reservoir of hydraulic fluid. An accumulator may also be provided for storing the high-pressure fluid.
In a water injection well, which is used to inject water or gas into the formation to assist in maintaining the pressure in the producing wells, the SCSSV be a simple spring-biased flapper valve, which is kept open by the injection flow itself. This arrangement eliminates the need for an SCSSV actuator altogether.
The present invention is directed to an apparatus for solving, or at least reducing the effects of, some or all of the aforementioned problems.